Apparatus is known in which the quantity of air flowing through the air intake of an internal combustion engine is measured by means of a sensing element whose resistance varies as a function of temperature. This sensing element is positioned in the airflow and its change in resistance corresponds to the quantity of airflowing thereby. The sensing element forms part of a bridge circuit. A regulating circuit increases the current through the sensing element until the bridge is rebalanced, that is the sensing element is kept at a substantially constant operating temperature. The change in current constitutes a measure of the quantity of airflowing past the sensing element, see, for example U.S. Pat. No. 3,747,577. This system has the disadvantage that deposits will accumulate on the sensing element during its operating life, thereby greatly decreasing the accuracy of the measurements. Further, under certain operating conditions of an internal combustion engine, it is possible that reversal in the direction of airflow take place, causing errors.